Commit 3cdf871a authored by Raymond Hettinger's avatar Raymond Hettinger

Neaten-up random module docs.

parent b2ddf797
...@@ -233,41 +233,18 @@ be found in any statistics text. ...@@ -233,41 +233,18 @@ be found in any statistics text.
parameter. parameter.
Alternative Generators: Alternative Generator:
.. class:: SystemRandom([seed]) .. class:: SystemRandom([seed])
Class that uses the :func:`os.urandom` function for generating random numbers Class that uses the :func:`os.urandom` function for generating random numbers
from sources provided by the operating system. Not available on all systems. from sources provided by the operating system. Not available on all systems.
Does not rely on software state and sequences are not reproducible. Accordingly, Does not rely on software state, and sequences are not reproducible. Accordingly,
the :meth:`seed` method has no effect and is ignored. the :meth:`seed` method has no effect and is ignored.
The :meth:`getstate` and :meth:`setstate` methods raise The :meth:`getstate` and :meth:`setstate` methods raise
:exc:`NotImplementedError` if called. :exc:`NotImplementedError` if called.
Examples of basic usage::
>>> random.random() # Random float x, 0.0 <= x < 1.0
0.37444887175646646
>>> random.uniform(1, 10) # Random float x, 1.0 <= x < 10.0
1.1800146073117523
>>> random.randint(1, 10) # Integer from 1 to 10, endpoints included
7
>>> random.randrange(0, 101, 2) # Even integer from 0 to 100
26
>>> random.choice('abcdefghij') # Choose a random element
'c'
>>> items = [1, 2, 3, 4, 5, 6, 7]
>>> random.shuffle(items)
>>> items
[7, 3, 2, 5, 6, 4, 1]
>>> random.sample([1, 2, 3, 4, 5], 3) # Choose 3 elements
[4, 1, 5]
.. seealso:: .. seealso::
M. Matsumoto and T. Nishimura, "Mersenne Twister: A 623-dimensionally M. Matsumoto and T. Nishimura, "Mersenne Twister: A 623-dimensionally
...@@ -280,6 +257,7 @@ Examples of basic usage:: ...@@ -280,6 +257,7 @@ Examples of basic usage::
random number generator with a long period and comparatively simple update random number generator with a long period and comparatively simple update
operations. operations.
Notes on Reproducibility Notes on Reproducibility
======================== ========================
...@@ -297,11 +275,34 @@ change across Python versions, but two aspects are guaranteed not to change: ...@@ -297,11 +275,34 @@ change across Python versions, but two aspects are guaranteed not to change:
sequence when the compatible seeder is given the same seed. sequence when the compatible seeder is given the same seed.
.. _random-examples:
Examples and Recipes Examples and Recipes
==================== ====================
Basic usage::
>>> random.random() # Random float x, 0.0 <= x < 1.0
0.37444887175646646
>>> random.uniform(1, 10) # Random float x, 1.0 <= x < 10.0
1.1800146073117523
>>> random.randrange(10) # Integer from 0 to 9
7
>>> random.randrange(0, 101, 2) # Even integer from 0 to 100
26
>>> random.choice('abcdefghij') # Single random element
'c'
>>> items = [1, 2, 3, 4, 5, 6, 7]
>>> random.shuffle(items)
>>> items
[7, 3, 2, 5, 6, 4, 1]
>>> random.sample([1, 2, 3, 4, 5], 3) # Three samples without replacement
[4, 1, 5]
A common task is to make a :func:`random.choice` with weighted probababilites. A common task is to make a :func:`random.choice` with weighted probababilites.
If the weights are small integer ratios, a simple technique is to build a sample If the weights are small integer ratios, a simple technique is to build a sample
...@@ -312,9 +313,9 @@ population with repeats:: ...@@ -312,9 +313,9 @@ population with repeats::
>>> random.choice(population) >>> random.choice(population)
'Green' 'Green'
A more general approach is to arrange the weights in a cumulative probability A more general approach is to arrange the weights in a cumulative distribution
distribution with :func:`itertools.accumulate`, and then locate the random value with :func:`itertools.accumulate`, and then locate the random value with
with :func:`bisect.bisect`:: :func:`bisect.bisect`::
>>> choices, weights = zip(*weighted_choices) >>> choices, weights = zip(*weighted_choices)
>>> cumdist = list(itertools.accumulate(weights)) >>> cumdist = list(itertools.accumulate(weights))
......
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